1. Field of the Invention
The invention relates to injection of a fluid into a gas by a nozzle. The invention is specifically directed to injection of a solution into a hot gas, where one specific use of the invention is in connection with NOx removal from exhaust gas from a diesel engine.
2. Description of Related Art
Nozzles are widely used for injection of one fluid into another, also in connection with NOx removal from engine exhaust gasses.
In WO 96/36797 patent application Hofmann et al. disclose injection of urea solution/air mixture into an exhaust gas in a vehicle. The mixture is sprayed into the exhaust gas pipe just inside the pipe close to the pipe wall so the solution does not become too hot before it is sprayed out in the gas. The distribution of the injected liquid is not uniform when seen from a cross section of the exhaust gas pipe.
Tsuneaki discloses another exhaust gas cleaning system in JP 2001152831. Here nitrogen oxides are reduced by urea solution sprayed into exhaust gas in a vehicle too. The injection nozzle is inserted through the wall of an exhaust gas pipe extending only a short distance from the inner surface, which creates an uneven flow pattern.
Kupper et al. disclose an injection nozzle for fuel in U.S. Pat. No. 4,502,196 comprising two coaxial tubes with a narrow space between filled with insulating material. The outer tube forms a hood around also the tip of the nozzle in order to keep the fuel below a temperature where formation of coke starts. The fuel is sprayed out through one hole only through both inner tube and hood in a coaxial direction with the nozzle.
Another gas cleaning method is described by Zauderer in U.S. Pat. No. 6,048,510. The content of nitrogen oxides in a flue gas in a coal fired boiler is reduced by injection of air atomised water droplets containing dissolved urea or ammonia. The droplets are injected where the temperature is optimal, and the size of the droplets is adjusted to this temperature by the nozzle construction.
However, these nozzles tend to block and none of the injections secure a very high degree of even distribution of the injected material.
It is the general object of this invention to provide a process and a nozzle for injection of an aqueous solution of a salt obtaining a stable and well distributed injection. The injection aimed at must be able to operate under all conditions without interruptions caused by blocking of the nozzle. Simultaneously, it must operate with a high degree of even distribution of the sprayed fluid in the gas, which it is sprayed into.
A process and a nozzle have now been found, so that when the nozzle is designed in accordance with the invention, a urea solution can be sprayed into exhaust gas from a vehicle obtaining a very even distribution in the exhaust gas. A nozzle of the invention operates for a long period without blocking the nozzles.